What is an Arachnoid Cyst?
An arachnoid cyst is a benign, fluid-filled cavity that forms in the space between the brain or spinal cord and the arachnoid layer — one of the protective membranes surrounding the central nervous system. These cysts are generally composed of cerebrospinal fluid (CSF).
Most arachnoid cysts are congenital (present at birth), while some are acquired after trauma, infection, or surgery. Many are asymptomatic and discovered incidentally on imaging. Large cysts or those compressing nearby neural structures can lead to neurological symptoms.
Classification of Arachnoid Cysts
Arachnoid cysts are categorized according to their anatomical position and the degree to which they affect neurological function:
1. Based on Location
Middle cranial fossa cysts: The most frequently encountered type, often located adjacent to the temporal lobe.
Posterior fossa (cerebellar) and supratentorial cysts: Found near the cerebellum or brainstem, possibly affecting coordination or cranial nerve function.
Spinal arachnoid cysts: These can form along the spinal canal and may lead to symptoms related to spinal cord or nerve root compression.
2. Galassi Classification (For Middle Cranial Fossa Cysts)
Type I: Small, usually asymptomatic.
Type II: Medium size, mild mass effect.
Type III: Substantial in size, these cysts exert marked pressure on adjacent brain tissue, often resulting in clinical symptoms.
Symptoms of Arachnoid Cyst
Most cysts are silent, but symptomatic ones can cause:
Headache (most common symptom)
Nausea and vomiting (from increased intracranial pressure)
Seizures
Balance problems or dizziness
Blurred or double vision (if compressing visual pathways)
In children: Head enlargement or developmental delay
Spinal arachnoid cysts may cause:
Back or neck pain
Weakness or numbness in the limbs
Difficulty with walking or coordination (gait disturbance)
Diagnostic Methods
1. Magnetic Resonance Imaging (MRI)
The most reliable tool for diagnosing arachnoid cysts. Cysts appear as CSF-like lesions without contrast enhancement and clearly show their size and effect on surrounding structures.
2. Computed Tomography (CT)
Useful to assess bony changes or adjacent skull remodeling. Often used in trauma-related cysts.
Treatment of Arachnoid Cysts
1. Observation (Conservative Management)
Asymptomatic, small cysts are usually monitored with periodic imaging.
Follow-up MRI is typically annual or symptom-driven.
2. Surgical Management
Indicated for symptomatic, large, or rapidly enlarging cysts:
Endoscopic fenestration: Involves forming a channel that allows the cyst’s contents to flow into the natural cerebrospinal fluid circulation system.
Minimally invasive and widely preferred.
Microsurgical fenestration or excision: Used for complex or multiloculated cysts.
Cystoperitoneal shunt placement: Diverts cyst fluid into the peritoneal cavity in selected cases.
Prognosis
Surgical outcomes are typically excellent.
Most asymptomatic cysts remain stable for life.